1. Field of the Invention
The present invention relates to monitoring and administering electric signals in living tissues. In particular, the present invention relates to a method and system for automatically optimizing multi-electrode pacing for cardiac resynchronization therapies.
2. Related Art
Cardiac resynchronization therapy (CRT) is an important new medical intervention for patients suffering from congestive heart failure. In congestive heart failure, symptoms develop due to the inability of the heart to function sufficiently well as a mechanical pump to supply blood to meet the body's physiological needs. Congestive heart failure is typically characterized by a gradual decline in cardiac function, punctuated by severe exacerbations leading eventually to death. Over five million patients in the United States are estimated to suffer from this malady.
The aim of cardiac resynchronization pacing is to induce the interventricular septum and the left ventricular free wall to contract at approximately the same time. Resynchronization therapy seeks to provide a contraction time sequence that can most effectively produce maximal cardiac output with minimal total energy expenditure by the heart. Generally, the optimal timing may be calculated by reference to hemodynamic parameters such as dP/dt, the first-order time derivative of the pressure waveform in the left ventricle. The dP/dt parameter is a well-documented proxy for left ventricular contractility.
In current practice, external ultrasound measurements are used to calculate the dP/dt parameter. An external ultrasound is used to observe wall motion directly. A clinician uses the ultrasound system in a tissue Doppler mode, which provides a feature known as tissue Doppler imaging (TDI), to evaluate the time course of displacement of the septum relative to the left ventricle free wall. Ultrasonographic evaluation using TDI or a similar approach can be an important tool for qualifying patients for CRT.
CRT as currently delivered is effective in about half to two-thirds of patients implanted with a resynchronization device. In approximately one third of these patients, CRT provides a two-class improvement in patient symptoms as measured based on the New York Heart Association (NYHA) fair level scale. In another one third, a one-class improvement in cardiovascular symptoms can be accomplished. In the remaining third, there is no improvement or, in a small minority, even a deterioration in cardiac performance. This third group of patients is referred to as non-responders. Furthermore, the NYHA one-class responders may experience only marginal or partial improvements, given the unfavorable results seen in the minority.
The synchronization therapy, in order to be optimal, targets the point of maximal delay on a cardiac wall segment, and advances the timing thereof to synchronize contraction with an earlier contracting region of the heart, such as the septum. The current placement technique for CRT devices is mostly empirical. A physician typically cannulates a vein that appears to be in the region described by the literature as most effective. He can then position the pacing device and perform stimulation. The lack of extra cardiac stimulation, such as diaphragmatic pacing, is confirmed. The current available techniques rarely provide sufficient time or means for optimizing the cardiac resynchronization process.
In general, CRT optimization must be performed through a laborious manual method performed by an ultrasonographer evaluating cardiac wall motion at different lead positions and different interventricular delay (IVD) settings. IVD refers to the difference between the timing of pacing pulses sent to different locations, such as the right ventricle and left ventricle. Pace makers can vary the atrio-ventricular delay, which is the delay between stimulations sent to the atria and the ventricle or ventricles. These timing settings are important in addition to the location of the pacing electrode in the left ventricle.
Some research efforts to assess cardiac motion through internal sensors have been made in the past. U.S. Pat. No. 6,625,493, entitled “Orientation of Patient's Position Sensor Using External Field,” to Kroll et al., filed on Aug. 24, 2001, discloses a positional accelerometer for indicating a patient's vertical acceleration. U.S. Pat. No. 6,002,963, entitled “Multi-Axial Accelerometer-Based Sensor for an Implantable Medical Device and Method of Measuring Motion Measurements There from,” to Mouchawar et al., filed on Feb. 17, 1995, discloses cardiac wall motion detection using an accelerometer. U.S. Pat. No. 5,991,661, entitled “System and Method for Measuring Cardiac Activity,” to Park et al., filed on Oct. 17, 1997, discloses the use of an accelerometer for rate-adaptive pacing. U.S. Pat. No. 6,044,299, entitled “Implantable Medical Device Having an Accelerometer,” to Nilsson, filed on Sep. 19, 1997, discloses an in-can accelerometer for providing rate control.
In addition to incorporating sensors in a pacing lead, a pacing lead can accommodate multiple pacing electrodes to provide flexibility in selecting pacing locations. U.S. Pat. No. 6,473,653, entitled “Selective Activation of Electrodes within an Implantable Lead,” to Schallhorn et al., filed on Mar. 2, 2000, discloses an implantable multi-electrode lead adapted to allow selective activation of the included electrodes to electrically excite the tissue in the vicinities of the activated electrodes. The Schallhorn system requires a controller for each electrode, and separate conductors for sending analog excitation pulses configuration commands to these controllers. Consequently, these controller circuits and conductors make the Schallhorn system bulky and unsuitable for many size-sensitive applications. U.S. Pat. No. 5,593,430, entitled “Bus System for Interconnecting an Implantable Medical Device with a Plurality of Sensors,” to Renger, filed on Jan. 27, 1995, discloses a two-conductor bus system for connecting physiologic sensors to a pacemaker. The two-conductor bus provides power to the sensors, and the sensors' output signals are modulated on the two wires.
With the flexibility of a multi-electrode lead comes the difficulty in selecting the optimal electrode and timing configurations, because the number of possible configurations increases geometrically with the number of available electrodes. Currently, TDI is the common technique for characterizing dyssynchrony with accuracy. TDI measures myocardial movement throughout the cardiac cycle and can quantify the timing differences between peak tissue movements in multiple areas of the left ventricle, which suggest the presence of dyssynchrony. TDI can also be used to determine cardiac wall positions via external ultrasonography, typically for purposes of measuring valve function, cardiac output, or rarely, synchrony index. However, TDI is a complex manual procedure and hence is time consuming and labor intensive. There are currently no clinically useful techniques for determining optimal CRT configurations on a substantially automatic or a real-time, machine-readable basis.
Hence, a need arises for a method for automated optimization of multi-electrode pacing for cardiac resynchronization.